I love Death and Company‘s Death & Co Modern Classic Cocktails book, and while there are many awesome sweetener recipes in it, the cinnamon bark syrup is the one I go to most often. It adds a spicy layer of depth to cocktails, particularly classics such as an old fashioned.
“In a saucepan, muddle 1 ounce of cassia cinnamon sticks until it’s broken up into shards. Add 2 cups of water and 2 cups of superfine sugar. Bring to a boil, stirring occasionally. Lower heat, cover, and simmer gently for 4 minutes. Remove from heat and let stand overnight. Strain through a cheesecloth-lined sieve.” — Death and Co: Modern Classic Cocktails
I made this yummy Grapefruit Gin Sour with it: